Computed Tomography (CT) is a common imaging technique based on measurements of X-ray attenuation in the scanned subject in multiplicity of projection angles. X-rays are attenuated by either absorption or scattering, in which case they may still reach the detector and be detected. Scatter radiation reduction and correction are required for both medical and nonmedical X ray CT imaging applications. Due to scattered X-ray photons, the local contrast and the data accuracy are deteriorated. Various methods to reduce and compensate for the scattering effect have been suggested and had being used. Early CT scanners have used narrow fan beams and were assisted by antiscatter grids so scatter radiation was not a major obstacle in these systems. However, in modern CT scanners, large area X-ray detectors having a plurality of detector element arranged in rows and columns are used to perform multi-slice imaging. In these multi-slice systems the width of the beam is significantly larger than in earlier single slice CT. Accordingly; the adverse effect of scattering is intensified. Further, some CT scanners use multiple sources of X rays associated with multiple detectors and the cross scattering increases the scattering problem.
In U.S. Pat. No. 5,666,391 to B. Ohnesorge, et al. (Sep. 9, 1997) the inventors suggest to correct for the scattered radiation by calculating a theoretical scattering distribution, based on the subject contour and shape as reconstructed from the data before the correction is applied, and subtracting the calculated scattering data from the actual collected data. The accuracy of this correction method is rather limited due to the high variability of scanned subjects.
In U.S. Pat. No. 6,618,466 to N. Ruola (Sep. 9, 2003), the inventor suggests positioning an array of shields between the radiation source and the scanned subject, acquiring some views of the subject from some angels wherein the shielded areas of the detector are used to measure the scattered radiation, removing the shields array and scanning the subject without the array. The scattering data collected in the first scan for certain parts of the detector and certain view angles are used to calculate the scattering map by interpolation for the entire array and for all view angles and then to correct the data of the second scan.
In U.S. Pat. No. 7,336,759 to N. Masatak (Feb. 26, 2008), the inventor suggests collecting in addition to the “main scan” to be corrected, which is done with a wide beam, additional few views with narrow beam. The data with narrow beam is used to assess the effect of the scattered radiation at the same projections, and to interpolate the scattering distribution from these measurements to the wide beam.
However, none of these methods provides a solution for accurate measurement and compensation for the scattered radiation without adding steps to the clinical procedure and without exposing the subject to additional radiation.
U.S. provisional patent application filed on 8 of May 2008 entitled “X RAY IMAGING SYSTEM WITH SCATTER RADIATION CORRECTION AND METHOD OF USING SAME” by the authors of the present invention, now submitted as PCT Application No. PCT/IL2009/000470, discloses an imaging system wherein radiation shield positioned in front of certain parts of the detector during subject scanning yield data which is useful to assess the scattered radiation and correct for it. In a CT scanner according to the said disclosure certain detector elements are shielded from radiation at all projection angles and do not contribute directly to the images, which might be undesirable.